The invention relates to a pulse-type sensor array beamforming apparatus for detection, localization, and/or imaging. It is particularly directed to pulse-type radar arrays employing radio-frequency (RF) transmissions, but may be applied to apparatus employing other types of transmissions.
Many radar applications require high resolution in range and angle, as well as flexible beam formation. Ballistic missile defense (BMD) constitutes one such application. In BMD, high resolution in range and angle are needed to separate, track, identify and classify objects. Flexible beam formation is needed to improve radar resource utilization, reduce revisit times, enhance angle estimation, and to improve robustness to interference.
In practice, there are several standard approaches to obtaining high resolution and flexible beam formation. High range resolution, for example, is obtained by transmitting a wide bandwidth waveform. On receive, the reflected energy is typically down-converted, sampled by an analog to digital converter (ADC), and pulse compressed.
Pulse compression is typically implemented in one of two ways. In the more traditional implementation, pulse compression is performed via digital matched filtering. When high range resolution is required, however, digital matched filtering can be difficult to implement. First of all, digital matched filtering requires that the ADC sample at a rate proportional to the transmitted signal""s bandwidth. High ADC sampling rates, however, can be difficult to achieve while simultaneously meeting high dynamic range requirements. Secondly, the matched filtering must be performed in real-time. Consequently, very high sampling rates may result in more data than can be processed in real-time by a designated processing device.
An alternative pulse compression implementation is called xe2x80x9cstretch processing.xe2x80x9d Stretch processing avoids the need for high rate digital sampling and processing. In stretch processing, the radar transmits a wideband linear frequency modulated (LFM) chirp waveform. On receive, the reflected energy is mixed with another LFM chirp, often called a xe2x80x9creplicaxe2x80x9d. The mixer output is then processed by an analog filter. Finally, the analog filter""s output is sampled and processed by a bank of narrowband digital compression filters (to resolve range). Note that the timing of the replica is usually set so that target signals at the output of the analog filter will be near baseband. The timing offsets between the replica chirp and target echoes will determine the frequencies at the output of the filter. As a result, the ADC sampling rate and subsequent digital processing rate do not need to be proportional to the transmitted signal bandwidth. Lower sampling rates simply limit the unambiguous range offset between the target echo and the replica.
High angular resolution, in contrast, is typically achieved by using a large antenna aperture. To enable high-speed beam scanning and flexible scan patterns, a phased array antenna is typically chosen. A phased array antenna is actually comprised of many individual antenna elements that are distributed spatially. Signals from the various antenna elements are combined to form beams.
The combining of antenna elements can be done using analog or digital methods. Of these, the digital approach (termed xe2x80x9cdigital beamformingxe2x80x9d) provides much greater flexibility. Digital beamforming necessitates that each antenna element (or subset of antenna elementsxe2x80x94known as a xe2x80x9csubarrayxe2x80x9d) is sampled. The digital outputs are then combined as desired, e.g., to steer the array""s gain toward a direction of interest.
To meet BMD requirements, the simultaneous utilization of stretch processing, a large phased array, and digital beamforming is desirable. However stretch-based pulse compression was originally formulated only for use with single-channel receivers. Currently, several methods exist for extending stretch processing to work with multi-channel phased array systems.
In one method, each RF antenna channel output is connected to a time delay unit (TDU). The delay value implemented by each TDU is chosen to steer the array toward a direction of interest. This process, known as time-delay beam steering, is well documented in the literature. The TDU""s effectively eliminate dispersion of signals coming from a chosen direction of interest, thus improving system performance in this direction. The outputs of the TDU""s are combined using an analog beamforming network. The output of this beamformer is connected to the input of a conventional (single channel) stretch processing receiver.
In a variation on this method, each TDU is directly connected to its own stretch processing receiver. The receiver outputs, which are digital, are then combined using a digital beamformer.
Each of these methods, and other similar variations, requires the utilization of multiple analog TDU""s. Typically, these units are costly. Size and weight may also be issues. Furthermore, the accuracy of these devices can be difficult to control and may ultimately limit performance. A method that does not rely on analog TDU""s is often desired.
In one such method, each RF antenna channel output is connected to a mixer. This device mixes the received energy with a replica chirp. The mixer output is then processed by an analog filter. Next, the analog filter output is sampled. Finally, the sampled signals are either (1) processed by a bank of narrowband digital compression filters (to resolve range), then digitally beamformed, or (2) digitally beamformed, then processed by a bank of narrowband digital compression filters (to resolve range).
In this method, as in conventional single channel stretch, the timing of the replica chirp is usually set so that the analog filter""s output (due to a hypothesized target position of interest) will be near baseband. However, since each channel processes signals at a different spatial location, it has been observed that the replica chirps on different channels should be time-delayed relative to one another, in a manner akin to time-delay beam steering.
This method, while partially eliminating the need for an analog TDU, can be shown to degrade as the time difference between the true signal""s position and the hypothesized target position (which is used to set the timing of the replica chirp) grows.
The invention is directed to further improvements in pulse-type sensor arrays for detection, localization, and/or imaging. For convenience, the invention will be described specifically in connection with radar arrays employing radio-frequency transmissions. The application to apparatus employing other types of transmissions will be understood by those skilled in the art.
In accordance with the invention, wideband chirp pulses are transmitted by the radar and reflected from objects to be detected, localized, and/or imaged. The reflected wideband echoes are received through each antenna of the antenna array. Subarray beamforming may then be performed, as desired. Each antenna channel (element or subarray, as applicable) output is then mixed with a delayed replica chirp, producing a signal in which the frequency slope is substantially reduced. Each resultant signal is filtered and sampled. The sampled signal is further digitally filtered to completely eliminate distortion (across the array) due to imprecise knowledge of object range. At this point, either (1) each signal is applied to a digital compression filter whose characteristic matches the reduced frequency slope, followed by digital beamforming, or (2) digital beamforming is performed, with each beamformed output applied to a digital compression filter whose characteristic matches the reduced frequency slope. With proper selection of parameters, the antenna gain, range and range resolution of known wideband-type radars can be obtained, while allowing the use of narrower band ADCs and digital compression filters, and without requiring precise knowledge of the object range. A radar display of the pulse-compressed, beamformed signals is provided.
More specifically, a method is provided for receiving, detecting, localizing and/or imaging signals. At each antenna channel, the propagation delay (relative to a common reference point) for signals from a selected direction is computed. The wideband echoes received through each antenna channel are mixed with a replica chirp. The replica chirp used by each channel""s receiver is effectively a delayed version of a common reference chirp. The delay offset value used by a given channel""s receiver is chosen to match the propagation delay experienced by a hypothesized signal traveling from the selected direction to that channel. This delay may be implemented in a variety of ways.
Next, an analog filter at the output of each mixer is used to reject all terms but the desired signal (a chirp who""s frequency sloped is substantially reduced) and to reduce the sampling rate of the subsequent sampling device.
Each filtered signal is then sampled. After sampling, each resultant digital signal is digitally filtered. The digital filters are chosen to effectively implement time delays. The delay used at a given channel is chosen to match the propagation delay for signals from the selected direction. This choice will completely eliminate distortion due to imprecise knowledge of true range (as long as the signal arrives from the selected direction). The delay filter may be implemented in a variety of ways. Notably, the delay filter may (optionally) be combined with other digital filtering operations (including the desired narrowband digital compression filter needed to resolve range).
After digitally filtering, the signal at each channel is combined with the signals from other channels via digital beamforming. The beams produced by this digital beamformer are then filtered using a narrowband digital compression filter to resolve range (if such filtering has not already been performed within each channel""s receiver).